Transcript

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SPINTRONICS INTRODUCTIONImagine a data storage device of the size of an atom working at the speed of light. Imagine amicroprocessor whose circuits could be changed on the fly. Imagine a computer memorythousands of times denser and faster than todays memories.The above mentioned things can be made possible with the help of spintronics.Spintronics is an amalgamation of Physics, Electronics and nanotechnology which deals withspin dependent properties of an electron instead of or in addition to its charge dependentproperties.Conventional electronics devices rely on the transport of electric charge carriers-electrons.But there are other dimensions of an electron other than its charge and mass i.e. spin. Thisdimension can be exploited to create a remarkable generation of spintronic devices. It isbelieved that in the near future, spintronics could be more revolutionary than any other thingthat nanotechnology has stirred up so far. WHY IS IT GOING TO BE ONE OF THE RAPIDLY EMERGING FIELDS?Moore’s law states that the number of transistors on a silicon chip doubles every 18 months.As there is rapid progress in the miniaturization of semiconductor electronic devices leads toa chip features smaller than 100 nm in size, device Engineers and Physicists will face variousproblems as to how to reduce the size of their devices without hampering the performance oftheir device. The solution of this problem of theirs would be given by spintronics.With the help of spintronics, complex yet compact devices can be easily manufactured. THE ELECTRON SPINOn close analysis of quantum theory the hydrogen atom, we find out that one of the solutionsof the Schrodinger wave equation is the spin of the electron. This takes values +1/2 and -1/2.If the electron rotates from west to east, it is denoted by ‘Spin Up’ and conversely, if theelectron rotates from east to west, the spin is denoted by ‘Spin Down’. In a magnetic field,electrons with "spin up" and "spin down" have different energies. Spintronic devices createspin-polarized currents and use the spin to control current flow.

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SPINTRONICS GIANT MAGNETO RESISTANCEMagnetism is the integral part of the present days data storage techniques. Right from thegramophone disks to the hard disks of the super computer, magnetism plays an importantrole. Data is recorded and stored as tiny areas of magnetized iron or chromium oxide. Toaccess the information, a read head detects the minute changes in magnetic field as the diskspins underneath it. In this way the read head detect the data and sends it to the varioussucceeding circuits. The magneto resistant devices can sense the changes in the magneticfield only to a small extent, which is appropriate to the existing memory devices. When wereduce the size and increase data storage density, we reduce the bits, so our sensor also has tobe small and maintain a very, very high sensitivity. This thought gave rise to the powerfuleffect called "GIANT MAGNETORESISTANCE". Giant magnetoresistance (GMR) cameinto picture in 1988, which lead the rise of spintronics. It results from subtle electron-spineffects in ultra-thin multilayer of magnetic materials, which causes huge changes in theirelectrical resistance when a magnetic field is applied. GMR is 200 times stronger thanordinary magnetoresistance. It was soon realized that read heads incorporating GMRmaterials would be able to sense much smaller magnetic fields, allowing the storage capacityof a hard disk to increase from 1 to 20 Gigabits.

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SPINTRONICS CONSTRUCTION OF GMRThe basic GMR device consists of a three-layer sandwich of a magnetic metal such as cobaltwith a nonmagnetic metal filling such as silver. Current passes through the layers consistingof spin-up and spin-down electrons. Those oriented in the same direction as the electron spinsin a magnetic layer pass through quite easily while those oriented in the opposite direction arescattered. If the orientation of one of the magnetic layers can easily be changed by thepresence of a magnetic field then the device will act as a filter, or spin valve, letting throughmore electrons when the spin orientations in the two layers are the same and fewer whenorientations are oppositely aligned. The electrical resistance of the device can therefore bechanged dramatically. In an ordinary electric current, the spin points at random and plays norole in determining the resistance of a wire or the amplification of a transistor circuit.Spintronic devices, in contrast, rely on differences in the transport of "spin up" and "spindown" electrons. When a current passes through the Ferro magnet, electrons of one spindirection tend to be obstructed. A Ferro magnet can even affect the flow of a current in anearby nonmagnetic metal. For example, in the present-day read heads in computer harddrives, wherein a layer of a nonmagnetic metal is sandwiched between two ferromagneticmetallic layers, the magnetization of the first layer is fixed, or pinned, but the secondferromagnetic layer is not. As the read head travels along a track of data on a computer disk,the small magnetic fields of the recorded 1s and 0`s change the second layers magnetizationback and forth parallel or antiparallel to the magnetization of the pinned layer. In the parallelcase, only electrons that are oriented in the favoured direction flow through the conductoreasily. In the antiparallel case, all electrons are impeded. The resulting changes in the currentallow GMR read heads to detect weaker fields than their predecessors; so that data can bestored using more tightly packaged magnetized spots on a disk.

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SPINTRONICS Important Applications of Spintronics:- MRAMAn important spintronic device, which is supposed to be one of the first spintronic devicesthat have been invented, is MRAM. Unlike conventional RAMs, MRAMs do not lose storedinformation once the power is turned off. Current generation PCs use SRAM and DRAM -both volatile memories. They can store information only if we have power. DRAM is a seriesof capacitors, a charged capacitor represents 1 where as an uncharged capacitor represents 0.To retain 1, you must constantly feed the capacitor with power because the charge you putinto the capacitor is constantly leaking out.MRAM is based on integration of magnetic tunnel junction (MTJ). Magnetic tunnel junctionis a three-layered device having a thin insulating layer between two metallic Ferro magnets.Current flows through the device by the process of quantum tunnelling; a small number ofelectrons manage to jump through the barrier even though they are forbidden to be in theinsulator. The tunnelling current is obstructed when the two ferromagnetic layers haveopposite orientations and is allowed when their orientations are the same. MRAM stores bitsas magnetic polarities rather than electric charges. When its polarity points in one direction, itholds1 and when its polarity points in other direction it holds 0. These bits need electricity tochange the direction but not to maintain them. MRAM is non-volatile so, even after shuttingdown the computer, all the bits retain the 1`s and 0`s.

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SPINTRONICS QUANTUM COMPUTERIn a quantum computer, the fundamental unit of information (called a quantum bit or qubit),is not binary but rather more quaternary in name. This qubit property arises as a directconsequence of its adherence to the laws of quantum mechanics. A qubit can exist not only ina state corresponding to the logical state 0 or 1 as in a classical bit, but also in statescorresponding to a blend or superposition of these classical states. In other words, a qubit canexist as a zero, a one or simultaneously as both 0 and 1, with a numerical coefficientrepresenting the probability for each state. Each electron spin can represent a bit; for instance,a 1 for spin up and 0 for spin down. With conventional computers, Engineers go to greatlengths to ensure that bits remain in stable, well-defined states. A quantum computer, incontrast, lies on encoding information within quantum bits, or qubits, each of which can existin a superposition of 0 and 1. By having a large number of qubits in superposition ofalternative states, a quantum computer intrinsically contains a massive parallelism.Unfortunately, in most physical systems, interactions with the surrounding environmentrapidly disrupt these superposition states. A typical disruption would effectively change asuperposition of 0 and 1 randomly into either a 0 or a 1, as process called decoherence. State-of-the-art qubits based on the charge of electrons in a semiconductor remain coherent for afew picoseconds at best and only at temperatures too low for practical applications. The rapiddecoherence occurs because the electric force between charges is strong and long range. Intraditional semiconductor devices, this strong interaction is beneficial, permitting delicatecontrol of current flow with small electronic fields. To quantum coherent devices, however, itis a disadvantage. As a result, an experiment was conducted on the qubits, which are based onthe electron-spin. Electron-spin qubits interact only weakly with the environment surroundingthem, principally through magnetic fields that are non-uniform in space or changing in time.Such fields can be effectively shielded.The goal of the experiment was to create some of these coherent spin states in asemiconductor to see how long they could survive. Much to the surprise, the optically excitedspin states in ZnSe remained coherent for several nanoseconds at low temperatures—1,000times as long as charge-based qubits. The states even survived for a few nanoseconds at roomtemperature. Subsequent studies of electrons in gallium arsenide (GaAs) have shown that,under optimal conditions, spin coherence in a semiconductor is possible.

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SPINTRONICS SPINTRONIC QUBITS1. In a conventional computer every bit has a definite value of 0 or 1. A series of eight bitscan represent any number from 0 to 255, but only one number at a time.2. Electron spins restricted to spin up and spin down could be used as bits.3. Quantum bits, or qubits, can also exist as superposition of 0 and 1, in effect being bothnumbers at once. Eight qubits can represent every number from 0 to 255 simultaneously.4. Electron spins are natural qubits; a tilted electron is a coherent superposition of spin up andspin down and is less fragile than other quantum electronic states.5. Qubits are extremely delicate: stray interactions with their surroundings degrade thesuperposition extremely quickly, typically converting them in to random ordinary bits.

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SPINTRONICSFIGURE 3-MRAM: A THREE LAYERED DEVICE HAVING A THIN INSULATED LAYER BETWEEN TWO FERRO MAGNETS

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SPINTRONICSFIGURE 4-BLOCH SPHERE: A GEOMETRICAL REPRESENTATION OF THE PURE STATE SPACE OF A TWO-LEVEL QUANTUM MECHANICAL SYSTEM (QUBIT) FIGURE 5-EXCITATON FROM GROUND STATE TO THE FIRST EXCITED STATE